Chlorination activation energies

The solutions are most stable above pH 11 where the decomposition rate is nearly independent of pH. In this region, the decomposition rate has a second-order dependence on the concentration of hypochlorite. It also increases with increa sing ionic strength. Thus concentrated solutions decompose much faster than dilute solutions. Because of an unusually high activation energy, the decomposition rate increases greatiy with temperature. Nevertheless, solutions with less than about 6% available chlorine and a pH above 11 have acceptable long-term stabiUty below about 30°C. 

Chlorination of Methane. Methane can be chlorinated thermally, photochemicaHy, or catalyticaHy. Thermal chlorination, the most difficult method, may be carried out in the absence of light or catalysts. It is a free-radical chain reaction limited by the presence of oxygen and other free-radical inhibitors. The first step in the reaction is the thermal dissociation of the chlorine molecules for which the activation energy is about 84 kj/mol (20 kcal/mol), which is 33 kJ (8 kcal) higher than for catalytic chlorination. This dissociation occurs sufficiendy rapidly in the 400 to 500°C temperature range. The chlorine atoms react with methane to form hydrogen chloride and a methyl radical. The methyl radical in turn reacts with a chlorine molecule to form methyl chloride and another chlorine atom that can continue the reaction. The methane raw material may be natural gas, coke oven gas, or gas from petroleum refining. 

There is, however, no direct evidence for the formation of Cl", and it is much more likely that the complex is the active electrophile. The substrate selectivity under catalyzed conditions ( t j = 160fcbenz) is lower than in uncatalyzed chlorinations, as would be expected for a more reactive electrophile. The effect of the Lewis acid is to weaken the Cl—Cl bond, which lowers the activation energy for o-complex formation. 

Important differences are seen when the reactions of the other halogens are compared to bromination. In the case of chlorination, although the same chain mechanism is operative as for bromination, there is a key difference in the greatly diminished selectivity of the chlorination. For example, the pri sec selectivity in 2,3-dimethylbutane for chlorination is 1 3.6 in typical solvents. Because of the greater reactivity of the chlorine atom, abstractions of primary, secondary, and tertiary hydrogens are all exothermic. As a result of this exothermicity, the stability of the product radical has less influence on the activation energy. In terms of Hammond s postulate (Section 4.4.2), the transition state would be expected to be more reactant-like. As an example of the low selectivity, ethylbenzene is chlorinated at both the methyl and the methylene positions, despite the much greater stability of the benzyl radical ... 

N NH3 detonates according to the equation 8I3N NH3 5Na + 9I2 + 6NH4I. When dry, the compd can expld without apparent external cause (Refs 10-15). It will expld in contact with coned acids, bromine, chlorine, ozone and hydrogen sulfide (Ref 16). The initiation or activation energy is 19.0 1.3kg-cal. Qf 35.0 kg-cal (Refs 8 9)... 

Kinetic studies have been carried out using the 1 1-complex iodobenzene dichloride as a source of molecular chlorine. In acetic acid solutions, the dissociation of this complex is slower than the rate of halogenation of reactive aromatics such as mesitylene or pentamethylbenzene, consequently the rate of chlorination of these is independent of the aromatic concentration. Thus at 25.2 °C first-order chlorination rate coefficients were obtained, being approximately 0.2 x 10-3 whilst the first-order dissociation rate coefficient was 0.16 xlO-3 from measurements at 25.2 and 45.6 °C the corresponding activation energies... 

Catalysis by hydrogen chloride or iodine monochloride in chlorination in carbon tetrachloride has also been examined. For the chlorination of pentamethylbenzene, the reaction was first-order in both aromatic and chlorine and either three-halves, or mixed first- and second-order in hydrogen chloride, but iodine monochloride was more effective as a catalyst and the chlorination of mesitylene was first-order in iodine monochloride the activation energy for this latter reaction (determined from data at 1.2 and 25.0 °C) was only 0.4 273. 

Trifluoroacetic acid has been examined as a solvent and chlorination of benzene in this is first-order in aromatic and chlorine, but for benzene a higher activation energy (11.4, determined from data at 25.0 and 45.4 °C) was obtained than for chlorination in carbon tetrachloride this unexpected result was attributed to an increase in desolvation energy of the reactants273. 

Notice that both steps of the chlorine-catalyzed reaction appear on the activation energy diagram. Each step of a mechanism has its own activation energy, so the diagram has two activation barriers. Experimental data indicate that the first barrier is higher than the second. Because the uncatalyzed reaction involves just one step, it has only... 

Evidently, the dissociation energies of the H—H and Cl—H bonds are very close and the triplet repulsion in the transition states of these reactions is, therefore, almost identical. Nevertheless, the quantities Eeo and re in these two reactions differ very considerably. The reason for this is that the H—H bond is nonpolar, while the Cl—H bond is polarized its AEA 92.3 kJ mol 1 (Equation [6.29]). As in the HC1 molecule, in the transition state there is evidently a strong attraction between Cl and H, which in fact induces a decrease in re and Ee0. If the Cl + H2 reaction was characterized by the same parameter re = 3.69 x 10-11m as the H + H2 reaction, an activation energy of Ee0 = 56.5 kJ mol 1 would be obtained for that reaction. The difference between the observed and expected activation energies (A ,ea = 36.7—56.5 = —19.8 kJ mol 1) must be attributed to the influence of the unequal electronegativities of the hydrogen and the chlorine atoms on Ec(, in the Cl + H2 reaction. 

The results of gas phase chlorination of hydrocarbons suggest that, due to differences in activation energy, tertiary radicals are more readily formed than secondary radicals which in turn are more readily formed than primary radicals. 

One further point deserves comment. Although reaction (65) is exothermic (see below) and probably has a low activation energy, it is inherently extremely complex. Indeed it would be surprising if the formation of the N N bond and the elimination of the chlorine atom and molecules occurred in a single step. Thus this reaction probably proceeds through a number of steps, e.g. 

Sampling rates at different temperatures have been determined by Huckins et al. (1999) for PAHs at 10,18, and 26 °C, by Rantalainen et al. (2000) for PCDDs, PCDFs, and non-ortho chlorine substituted PCBs at 11 and 19 °C, and by Booij et al. (2003a) for chlorobenzenes, PCBs, and PAHs at 2,13 and 30 °C. The effect of temperature on the sampling rates can be quantified in terms of activation energies (A a) for mass transfer, as modeled by the Arrhenius equation... 

As seen from Table 4.2, activation energies of chlorine substitution in nitrochlorobenzenes under the action of diverse nucleophilic reagents are in agreement with a, of anion-radicals. Constants and of 4-chloronitrobenzene anion-radical are close to the and constants of nitrobenzene... 

Constants HFC and Activation Energy in the Ethoxyl Group Substitution for Chlorine When Treating Nitro- and Dinitrochlorobenzenes with a Mixture of Ethyl Alcohol and Piperidine... 

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